Mr Mousavi said the revolution had failed to achieve its goals
Iran's opposition leader Mir Hossein Mousavi has said he will continue his struggle against the government.
In a statement posted on his website, Mr Mousavi said the 1979 Islamic revolution had failed to achieve most of its goals.
He said politically motivated arrests of protesters were illegal and more should be done to secure people's rights.
His comments constitute one of his strongest challenges to the government.
They also come at a particularly sensitive time. Iran will mark the 31st anniversary of the founding of the Islamic Republic on 11 February.
As one of the key players in the founding of the Islamic Republic, his comments have extra resonance.
Jon Leyne, the BBC's Tehran correspondent who is reporting from London, says Mr Mousavi's comments will outrage hard-line supporters of the government. He is now pushing to the very limit of what he can say without being arrested.
Call for rallies
The anniversary is one of the most important dates in Iran's political calendar. Mr Mousavi and his reformist ally Mehdi Karroubi have called on their supporters to attend rallies on the day of the anniversary.
But hardliners, including the country's supreme leader, have warned that anti-government protests will not be tolerated.
Jon Leyne, BBC Tehran correspondent, reporting from London
Mir Hossein Mousavi's comments will be seen as highly inflammatory by hard-line supporters of the government. They will be infuriated by his suggestion that Iran is suffering a tyranny similar to that under the shah.
In calling for changes to the constitution, Mr Mousavi seems to be moving closer to the position of his increasingly radical supporters, who are beginning to challenge not only the election result, but the very system itself.
It is as if Mr Mousavi is now challenging the government to arrest him - a move the authorities have, until now, decided would be a step too far.
The government's language and actions are also getting progressively tougher. An official has said that nine more opposition protesters, or rioters as the government terms them, will soon be executed, following two similar executions last week.
Everything is building up towards a major confrontation on the 11 February, the anniversary of the Iranian Revolution that overthrew the shah.
Tensions in Iran are still high after bloody demonstrations during the Shia ritual of Ashura in December when eight protesters were killed and officials said over 1,000 were arrested.
Mr Mousavi's nephew Seyed Ali Mousavi was among those killed.
The December clashes were the worst episode of violence since the aftermath of last June's disputed presidential election and subsequent government crackdown.
On Tuesday the government threatened to execute nine people who were allegedly arrested during the post-election unrest that erupted after the vote that returned President Mahmoud Ahmadinejad to power.
"The green movement will not abandon its peaceful fight... until people's rights are preserved," Mr Mousavi wrote on his Kalemeh website. "Peaceful protests are Iranians' right."
Mr Mousavi also said that the Islamic revolution in Iran had failed to eradicate the "roots of tyranny and dictatorship" that, he said, marked the shah's era.
He said he no longer believed, as he once did, "that the revolution had removed all those structures which could lead to totalitarianism and dictatorship".
"Today, one can identify both elements and foundations which produce dictatorship as well as resistance against returning to this dictatorship," he said.
"Stifling the media, filling the prisons and brutally killing people who peacefully demand their rights in the streets indicate the roots of tyranny and dictatorship remain from the monarchist era. I don't believe that the revolution achieved its goals," Mr Mousavi added.
Last month Mr Mousavi said that he was not afraid to die for the cause of reform.